The Effect of the Subglacial Water Pressure on the Sliding Velocity of a Glacier in an Idealized Numerical Model

Abstract

ABSTRACT. In order to interpret observed short-term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model. In particular the transient stages of grow ing or shrinking water-filled cavities at the icebedrock interface were analysed. It was found that the sl iding veloci ty was larger when cavities were growing than when they had reached the steady-state size for a given water pressure. The smallest sliding velocities occurred while cavi ties were shrinking. When cavitation is substantial a small drop of water pressure below the steady-state value (e.g. by 0.5 bar) can temporarily cause backward sliding. A limiting water pressure at which sliding becomes unstable is derived. The consequences of more realistic assumptions than those of the model are discussed. RESUME. Injluence de la pression hydrauligue sous-glaciaire sur la vitesse de glissement d'un glacier dans un modete numerigue idealise. Dans le but d'interpreter les variations de vitesse a court terme qui ont ete observees, on a etudie a l'aide d'un modele numerique idealise l'effet exerce par une pression hydraulique sous-glaciaire variable sur la vitesse de glissement d'un glacier. On a etudie en particulier les etats transitoires de la croissance et de la decroissance des poches d'eau situees a l'interface glace-bedrock. II s'est revele que pendant la phase de croissance des cavites, la vitesse de glissement est plus grande qu'a l'etat stationnaire pour une pression hydraulique donnee. Les vitesses de g lissement les plus faibl es se produisent lors du retrecissement des cavites. Lorsque le volume des cavites est suffisamment grand, une faible diminution de la pression (p.ex. de 0,5 bar) peut temporairement provoquer un glissement vers l'amont. On a determine une valeur limite de la pression hydraulique pour laquelIe de glissement devient instable. On discute les consequences d'hypotheses plus realistes que celIes sur lesquelles le modele repose. ZUSAMMENFASSUNG. Der Einjluss des suhglazialen Wasserdruckes au] die Gleitgeschwindigkeit eines Gletsehers, untersueht mit einem idealisierten numerischen Modell.Mit dem Ziel, beobachtete kurzzeitige Geschwindigkeitsschwankungen zu erklaren, wurde der EinAuss eines veranderli chen subglazialen Wasserdruckes auf die Gleitgeschwindigkeit mit einem idealisierten numerischen Modell untersucht. Insbesondere wurden die Ubergangsphasen von wachsenden od er sch rumpfenden, wassergefUllten Hohlraumen an der GrenzAache von Eis und Felsbett analysiert. Es zeigte sich, dass die Gleitgeschwindigkeit grosser war, wenn Hohlrliume zu wachsen begannen, als wenn sie die fUr einen bestimmten Wasserdruck endgUltige Grosse erreicht hatten. Die kleinsten Gleitgeschwindigkeiten kamen vor, wlihrend die Hohlraume schrumpften. Bei genugendem Ausmass der H ohlraumbi ldu ng kann ein geringfUgiges Absinken des Wasserdruckes unter den Gleichgewichtswert (z.B. urn 0,5 bar) vorUbergehend Ruckwlirtsgleiten verursachen. Ein Gr...

Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

“…We think that this source is cavitation at the glacier bed, caused by the high basal water pressures, and promoted additionally by the high sliding velocity that develops in the mini-surges. This interpretation is particularly supported by the fact that the maximum sliding velocity always coincides with the maximum uplift rate (section 4), a relationship that is characteristic of basal cavitation as shown by the calculations of Iken (1981). The lag between the abrupt rise in water pressure is interpreted as the time for growth of the basal cavities once the high water pressure is applied.…”

Surge dynamics in the Nathorstbreen glacier system, Svalbard

“…The somewhat higher amount of water available at the bed influences the basal motion (e.g. Iken, 1981;Bartholomaus et al, 2008). A surge nucleus is formed and an initial drawdown occurs and (Budd, 1975;McMeeking and Johnson, 1986).…”

Propagating Strain Anomalies during Mini-Surges of Variegated Glacier, Alaska, U.S.A.

“…Based on the observed correlation of horizontal velocity with surface uplift, theoretical models (Iken, 1981) and earlier concepts about glacier sliding (Lliboutry, 1965), they hypothesized that the increased speed they observed was caused by the action of water in basal cavities. They also inferred that the pressure of the water was of principal importance through its effect on the opening or closing rate of basal cavities and that the amount of basal water present was of secondary importance.…”

“…We address these questions using strain measurements on Variegated Glacier during the early part of the melt seasons of 1979, 1980, and 1981. Variegated Glacier surged in 1982(Kamb and others, 1985, and the observations also provide information about the glacier's behavior just prior to surge initiation.…”